-
Mendel's Paper in English
Experiments in
Plant Hybridization (1865)
by Gregor
Mendel
Read at the meetings of February
8th, and March 8th, 1865
[1] Introductory Remarks
Experience of artificial fertilization,
such as is effected with
ornamental
plants in order to
obtain new
variations in color, has led to
the
experiments
which
will
here
be
discussed.
The
striking
regularity
with
which
the
same
hybrid
forms
always
reappeared
whenever
fertilization
took
place between the same
species induced further experiments to be
undertaken, the object
of
which was
to
follow up the
developments
of the
hybrids in their progeny.
To this object numerous careful
observers, such as K?lreuter, G?rtner,
Herbert, Lecoq, Wichura and others,
have devoted a part of their lives
with
inexhaustible perseverance. G?rtner especially in
his work
Die
Bastarderzeugung im
Pflanzenreiche
, has recorded very
valuable
observations; and quite
recently Wichura published the results of some
profound
investigations
into
the
hybrids
of
the
Willow.
That, so
far,
no
generally applicable law governing the
formation and development of
hybrids
has been successfully formulated can hardly be
wondered at by
anyone who is acquainted
with the extent
of the task,
and can appreciate
the
difficulties with which experiments of this class
have to contend.
A final decision can
only be arrived at when we shall have before us
the
results of detailed experiments
make on plants belonging to the most
diverse orders.
Those who
survey the work done in this department will
arrive at the
conviction
that
among
all
the
numerous
experiments
made,
not
one
has
been
carried out to such an
extent and in such a way as to make it possible
to determine the number of different
forms under which the offspring of
the
hybrids appear, or to arrange these forms with
certainty according
to their separate
generations, or definitely to ascertain their
statistical relations.
第
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It
requires
indeed
some
courage
to
undertake
a
labor
of
such
far-
reaching
extent; this appears, however,
to be the only right way by which we can
finally reach the solution of a
question the importance of which cannot
be overestimated in connection with the
history of the evolution of
organic
forms.
The
paper
now
presented
records
the
results
of
such
a
detailed
experiment.
This experiment
was practically confined to a small plant group,
and is
now,
after
eight
years'
pursuit,
concluded
in
all
essentials.
Whether
the
plan upon which the
separate experiments were conducted and carried
out
was the best suited to attain the
desired end is left to the friendly
decision of the reader.
[2]
Selection of the Experimental Plants
The value and utility
of any
experiment
are determined by
the fitness of
the material
to the purpose for which it is used, and thus in
the case
before
us
it
cannot
be
immaterial
what
plants
are
subjected
to
experiment
and in what
manner such experiment is conducted.
The
selection
of
the
plant
group
which
shall
serve
for
experiments
of
this
kind must
be made with all possible care if it be desired to
avoid from
the outset every risk of
questionable results.
The experimental
plants must necessarily:
1.
Possess constant differentiating
characteristics.
2.
The hybrids of such plants must, during
the flowering period, be
protected from
the influence of all foreign pollen, or be easily
capable of such protection.
The hybrids and their offspring should
suffer no marked disturbance in
their
fertility in the successive generations.
Accidental impregnation by foreign
pollen, if it occurred during the
experiments and were not recognized,
would lead to entirely erroneous
conclusions.
Reduced
fertility
or
entire
sterility
of
certain
forms,
such
as
occurs
in
the
offspring
of
many
hybrids,
would
render
the
experiments
very difficult
or entirely frustrate them. In order to discover
the
relations in which the hybrid forms
stand towards each other and also
towards their progenitors it appears to
be necessary that all member of
the
series developed in each successive generations
should be,
without
exception
, subjected to
observation.
第
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At the very outset special attention
was devoted to the
Leguminosae
on
account of their peculiar floral
structure. Experiments which were made
with
several
members
of
this
family
led
to
the
result
that
the
genus
Pisum
was found
to possess the necessary qualifications.
Some
thoroughly
distinct
forms
of
this
genus
possess
characters
which
are
constant, and easily and certainly
recognizable, and when their hybrids
are mutually crossed they yield
perfectly fertile progeny. Furthermore,
a disturbance through foreign pollen
cannot easily occur, since the
fertilizing organs are closely packed
inside the keel and the anthers
burst
within
the
bud,
so
that
the
stigma
becomes
covered
with
pollen
even
before the flower opens. This
circumstance is especially important. As
additional advantages worth mentioning,
there may be cited the easy
culture of
these plants in the open ground and in pots, and
also their
relatively
short
period
of
growth.
Artificial
fertilization
is
certainly
a
somewhat
elaborate
process,
but
nearly
always
succeeds.
For
this
purpose
the
bud is opened
before it
is
perfectly developed, the keel is
removed,
and
each
stamen
carefully
extracted
by
means
of
forceps,
after which
the
stigma can at once be dusted over with
the foreign pollen.
In all, 34 more or
less distinct varieties of Peas were obtained from
several seedsmen and subjected to a two
year's trial. In the case of one
variety there were noticed, among a
larger number of plants all alike,
a
few forms which were markedly different. These,
however, did not vary
in
the
following
year,
and
agreed
entirely
with
another
variety
obtained
from the same
seedsman; the seeds were therefore doubtless
merely
accidentally mixed. All the
other varieties yielded perfectly constant
and
similar
offspring;
at
any
rate,
no
essential
difference
was
observed
during two trial years. For
fertilization 22 of these were selected and
cultivated during the whole period of
the experiments. They remained
constant
without any exception.
Their
systematic classification is difficult and
uncertain. If we adopt
the strictest
definition of a species, according to which only
those
individuals belong to a species
which under precisely the same
circumstances display precisely similar
characters, no two of these
varieties
could be referred to one species. According to the
opinion of
experts,
however,
the
majority
belong
to
the
species
Pisum
sativum;
while
the
rest
are
regarded
and
classed,
some
as
sub-species
of
P.
sativum,
and
some
as independent species, such as P. quadratum, P.
saccharatum, and
P. umbellatum. The
positions, however, which may be assigned to them
in
a classificatory system are quite
immaterial for the purposes of the
experiments
in
question.
It
has
so
far
been
found
to
be
just
as
impossible
to draw a sharp
line between the hybrids of species and varieties
as
between species and varieties
themselves.
第
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[3] Division and Arrangement of the Experiments
If two plants which differ constantly
in one or several characters be
crossed, numerous experiments have
demonstrated that the common
characters
are transmitted unchanged to the hybrids and their
progeny;
but
each
pair
of
differentiating
characters,
on
the
other
hand,
unite
in
the hybrid to form a new character,
which in the progeny of the hybrid
is
usually variable. The object of the experiment was
to observe these
variations in the case
of each pair of differentiating characters, and
to
deduce
the
law
according
to
which
they
appear
in
successive
generations.
The experiment
resolves itself therefore into just as many
separate
experiments are there are
constantly differentiating characters
presented in the experimental plants.
The various forms of Peas selected for
crossing showed differences in
length
and color of the stem; in the size and form of the
leaves; in the
position,
color, size of
the flowers;
in the length
of the flower
stalk;
in
the
color,
form,
and
size
of
the
pods;
in
the
form
and
size
of
the
seeds;
and in the color
of the
seed-coats
and of the
albumen [cotyledons]. Some
of the
characters noted
do not permit
of a
sharp and certain
separation,
since the
difference is of a
difficult to define.
Such characters could not be utilized for the
separate experiments; these could only
be applied to characters which
stand
out clearly and definitely in the plants. Lastly,
the result must
show
whether
they,
in
their
entirety,
observe
a
regular
behavior
in
their
hybrid
unions,
and
whether
from
these
facts
any
conclusion
can
be
reached
regarding those
characters which possess a subordinate
significance in
the type.
The characters which were selected for
experiment relate:
1.
To the
difference in the
form of the ripe seeds.
These are
either
round or roundish, the
depressions, if any, occur on the surface,
being always only shallow; or they are
irregularly angular and
deeply wrinkled
(P. quadratum).
2.
To
the
difference
in
the
color
of
the
seed
albumen
(endosperm).
The
albumen
of
the
ripe
seeds
is
either
pale
yellow,
bright
yellow and
orange
colored,
or
it
possesses
a
more
or less
intense
green
tint.
This
difference
of
color
is
easily
seen
in
the
seeds
as
their
coats
are transparent.
3.
To
the
difference
in
the
color
of
the
seed-coat.
This
is
either
white,
with which character
white flowers are constantly correlated; or
it is gray, gray-brown, leather-brown,
with or without violet
spotting,
in
which
case
the
color
of
the
standards
is
violet,
that
第
4
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57
页
of the wings purple, and the stem in
the axils of the leaves is of
a reddish
tint. The gray seed-coats become dark brown in
boiling
water.
4.
To the
difference in the form of the ripe
pods.
These are either
simply inflated, not contracted in
places; or they are deeply
constricted
between the seeds and more or less wrinkled (P.
saccharatum).
5.
To the
difference in the
color of the unripe pods.
They are either
light
to
dark
green,
or
vividly
yellow,
in
which
coloring
the
stalks,
leaf-veins, and
calyx participate.*
6.
To the
difference in the
position of the flowers.
They are
either
axial, that is, distributed
along the main stem; or they are
terminal,
that
is,
bunched
at
the
top
of
the
stem
and
arranged
almost
in a false umbel; in this case the
upper part of the stem is more
or less
widened in section (P. umbellatum).
7.
To
the
difference
in
the
length
of
the
stem.
The
length
of
the
stem
is
very
various
in
some
forms;
it
is,
however,
a
constant
character
for each, in so far that healthy
plants, grown in the same soil,
are
only subject to unimportant variations in this
character. In
experiments
with
this
character,
in
order
to
be
able
to
discriminate
with
certainty,
the
long
axis
of
6
to
7
ft.
was
always
crossed
with
the
short one of 3/4 ft. to 1 and 1/2 ft.
Each two of the differentiating
characters enumerated above were united
by cross-fertilization. There were made
for the
1st trial
2nd trial
3rd trial
4th trial
5th trial
6th trial
7th trial
60 fertilizations
on 15 plants.
58 fertilizations on 10
plants.
35 fertilizations on 10 plants.
40 fertilizations on 10 plants.
23 fertilizations on 5 plants.
34 fertilizations on 10 plants.
37 fertilizations on 10 plants.
*One
species
possesses
a
beautifully
brownish-red
colored
pod,
which
when
ripening turns to violet and blue.
Trials with this character were only
begun last year.
From
a
larger
number
of
plants
of
the
same
variety
only
the
most
vigorous
were chosen for fertilization. Weakly
plants always afford uncertain
results,
because
even
in
the
first
generation
of
hybrids,
and still
more
so in the subsequent
ones,
many
of the
offspring either entirely
fail to
flower or only form
a few and inferior seeds.
第
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Furthermore, in
all the experiments reciprocal crossings were
effected
in such a way that each of the
two varieties which in one set of
fertilizations served as seed-bearer in
the other set was used as the
pollen
plant.
The plants were grown in garden
beds, a few also in pots, and were
maintained
in
their
natural
upright
position
by
means
of
sticks,
branches
of
trees,
and
strings
stretched
between.
For
each
experiment
a number
of
pot
plants
were
placed
during
the
blooming
period
in
a
greenhouse,
to
serve
as
control
plants
for
the
main
experiment
in
the
open
as
regards
possible
disturbance by
insects. Among the insects which visit Peas the
beetle
Bruchus
pisi
might
be
detrimental
to
the
experiments
should
it appear
in
numbers.
The
female
of
this
species
is
known
to
lay
the
eggs
in
the
flower,
and
in
so
doing
opens
the
keel;
upon
the
tarsi
of
one
specimen,
which
was
caught
in
a
flower,
some
pollen
grains
could
clearly
be
seen
under
a
lens.
Mention must also be made
of
a circumstance
which
possibly might
lead to
the
introduction
of
foreign
pollen.
It
occurs,
for
instance,
in
some
rare
cases
that
certain
parts
of
an
otherwise
normally
developed
flower
wither,
resulting in a partial exposure of the
fertilizing organs. A defective
development
of
the
keel
has
also
been
observed,
owing
to
which
the
stigma
and anthers remained
partially covered. It also sometimes happens that
the pollen does not reach full
perfection. In this event there occurs a
gradual lengthening of the pistil
during the blooming period, until the
stigmatic tip protrudes at the point of
the keel. This remarkable
appearance
has also been observed in hybrids of Phaseolus and
Lathyrus.
The
risk
of
false
impregnation
by
foreign
pollen
is,
however,
a
very
slight
one
with
Pisum,
and
is
quite
incapable
of disturbing
the
general
result.
Among more than
10,000 plants which were carefully examined there
were
only
a
very
few
cases
where
an
indubitable
false
impregnation
had
occurred.
Since in the greenhouse such a case was
never remarked, it may well be
supposed
that
Bruchus
pisi
,
and
possibly
also
the
described
abnormalities
in the floral structure, were to blame.
[4] The Forms of the Hybrids
Experiments
which
in
previous
years
were
made
with
ornamental
plants
have
already affording
evidence that the hybrids, as a rule, are not
exactly
intermediate
between
the
parental
species.
With
some
of
the
more
striking
characters,
those,
for
instance,
which
relate
to
the
form
and
size
of
the
leaves, the pubescence of the several
parts, etc., the intermediate,
indeed,
is nearly always to be seen; in other cases,
however, one of the
two
parental
characters
is
so
preponderant
that
it
is
difficult,
or
quite
impossible, to detect the other in the
hybrid.
第
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This is precisely the case with the Pea
hybrids. In the case of each of
the
7
crosses
the
hybrid-character
resembles
that
of
one
of
the
parental
forms
so
closely
that
the
other
either
escapes
observation
completely
or
cannot be detected with
certainty. This circumstance is of great
importance in the determination and
classification of the forms under
which
the
offspring
of
the
hybrids
appear.
Henceforth
in
this
paper
those
characters which are
transmitted entire, or almost unchanged in the
hybridization, and therefore in
themselves constitute the characters of
the
hybrid,
are
termed
the
dominant
,
and
those
which
become
latent
in
the
process
recessive
.
The
expression
has
been
chosen
because
the
characters thereby designated withdraw
or entirely disappear in the
hybrids,
but nevertheless reappear unchanged in their
progeny, as will
be demonstrated later
on.
It was furthermore shown by the
whole of the experiments that it is
perfectly immaterial whether the
dominant character belongs to the seed
plant or to the pollen plant; the form
of the hybrid remains identical
in
both
cases.
Thi
s
interesting
fact
was
also
emphasized
by
G?rtner,
with
the
remark that even the most practiced expert is not
in a position to
determine in a hybrid
which of the two parental species was the seed or
the pollen plant.
Of
the
differentiating
characters
which
were
used
in
the
experiments
the
following are dominant:
1.
The round or
roundish form of the seed with or without shallow
depressions.
2.
The yellow coloring of the seed albumen
[cotyledons].
3.
The gray, gray-brown, or leather brown
color of the seed-coat, in
association
with
violet-red
blossoms
and
reddish
spots
in
the
leaf
axils.
4.
The simply
inflated form of the pod.
5.
The green
coloring of the unripe pod in association with the
same
color of the stems, the leaf-veins
and the calyx.
6.
The distribution of the flowers along
the stem.
7.
The
greater length of stem.
With regard to
this last character it must be stated that the
longer of
the two parental stems is
usually exceeded by the hybrid, a fact which
is
possibly
only
attributable
to
the
greater
luxuriance
which
appears
in
all
parts
of
plants
when
stems
of
very
different
lengths
are
crossed.
Thus,
for
instance,
in
repeated
experiments,
stems
of
1
ft.
and
6
ft.
in
length
yielded without exception hybrids which
varied in length between 6 ft.
and 7
[and] 1/2 ft.
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The
hybrid
seeds
in
the
experiments
with
seed-coat
are
often
more
spotted,
and the spots
sometimes coalesce into small bluish-violet
patches. The
spotting also frequently
appears even when it is absent as a parental
character.
The hybrid forms
of the
seed-shape
and of the
[color of the] albumen
are
developed immediately after the
artificial fertilization by the mere
influence of the foreign pollen. They
can, therefore, be observed even
in
the
first
year
of
experiment,
whilst
all
the
other
characters
naturally
only
appear
in
the
following
year
in
such
plants
as
have
been
raised
from
the crossed seed.
[5] The First Generation From the
Hybrids
In
this
generation
there
reappear,
together
with
the
dominant
characters,
also
the
recessive
ones
with
their
peculiarities
fully
developed,
and
this
occurs in the
definitely expressed average proportion of 3:1, so
that
among each 4 plants of this
generation 3 display the dominant character
and one the recessive. This relates
without exception to all the
characters
which were investigated in the experiments. The
angular
wrinkled
form
of
the
seed,
the
green
color
of
the
albumen,
the
while
color
of the seed-coats and the flowers, the
constrictions of the pods, the
yellow
color of the unripe pod, of the stalk, of the
calyx, and of the
leaf
venation,
the
umbel-like
form
of
the
inflorescence,
and
the
dwarfed
stem,
all reappear in the numerical proportion given,
without any
essential alteration.
Transitional forms were not observed in
any
experiment.
Since
the
hybrids
resulting
from
reciprocal
crosses
are
formed
alike
and
present no appreciable
difference in their subsequent development,
consequently these results can be
reckoned together in each experiment.
The
relative
numbers
which
were
obtained
for
each
pair
of
differentiating
characters
are as follows:
Expt.
1.
Form
of
seed.
--
From
253
hybrids
7324
seeds
were
obtained
in the second trial
year. Among them were 5474 round or roundish
ones
and
1850
angular
wrinkled
ones.
Therefrom
the
ratio
2.96:1
is
deduced.
?
Expt. 2. Color
of albumen. -- 258 plants yielded 8023 seeds, 6022
yellow, and 2001 green; their ratio,
therefore, is as 3.01:1.
?
In these two experiments
each pod yielded usually
both kinds of
seed. In
well-developed
pods
which
contained
on
the
average
6
to
9
seeds,
it
often
happened
that
all
the
seeds
were
round
(Expt.
1)
or
all
yellow
(Expt.
2);
第
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57
页
on
the
other
hand
there
were
never
observed
more
than
5
wrinkled
or
5
green
ones on one pod. It
appears to make no difference whether the pods are
developed early or later in the hybrid
or whether they spring from the
main
axis or from a lateral one. In some few plants
only a few seeds
developed in the first
formed pods, and these possessed exclusively one
of
the
two
characters,
but
in
the
subsequently
developed
pods
the
normal
proportions were maintained
nevertheless.
As in separate pods, so
did the distribution of the characters vary in
separate
plants.
By
way
of
illustration
the
first
10
individuals
from
both
series of experiments
may serve.
Experiment 1
Experiment 2
Form of Seed
Color of Albumen
Plants Round Angular
Yellow
Green
1 45 12
25 11
2 27
8
32 7
3 24 7
14 5
4
19 10
70 27
5 32 11
24 13
6 26 6
20 6
7
88 24
32 13
8 22 10
44 9
9 28
6
50 14
10 25 7
44
18
As extremes in the
distribution of
the
two seed characters
in one
plant,
there
were
observed
in
Expt.
1
an
instance
of
43
round
and
only
2
angular,
and
another
of
14
round
and
15
angular
seeds.
In
Expt.
2
there was
a
case
of
32
yellow
and
only
1
green
seed,
but
also
one
of
20
yellow
and
19
green.
These
two
experiments
are
important
for
the
determination
of
the
average
ratios, because with a smaller number
of experimental plants they show
that
very
considerable
fluctuations
may
occur.
In
counting
the
seeds,
also,
especially
in
Expt.
2,
some
care
is
requisite,
since
in
some
of
the
seeds
of many
plants the green color of the albumen is less
developed, and at
first may be easily
overlooked. The cause of this partial
disappearance
of
the
green
coloring
has
no
connection
with
the
hybrid-
character
of
the
plants, as it likewise occurs in the
parental variety. This peculiarity
is
also
confined
to
the
individual
and
is
not
inherited
by
the
offspring.
In
luxuriant
plants
this
appearance
was
frequently
noted.
Seeds
which
are
damaged
by
insects
during
their
development
often
vary
in
color
and
form,
but with a little practice in sorting,
errors are easily avoided. It is
almost
superfluous
to
mention
that
the
pods
must
remain
on
the
plants
until
they are thoroughly
ripened
and have become
dried, since it
is only then
that the shape and color of the seed
are fully developed.
第
9
页
共
57
页
?
?
?
?
?
Expt. 3. Color of the seed-coats. --
Among 929 plants, 705 bore
violet-red
flowers
and
gray-
brown
seed-coats;
224
had
white
flowers
and white seed-coats, giving the
proportion 3.15:1.
Expt. 4. Form of
pods. -- Of 1181 plants, 882 had them simply
inflated, and in 299 they were
constricted. Resulting ratio,
2.95:1.
Expt. 5. Color of the unripe pods. --
The number of trial plants
was 580, of
which 428 had green pods and 152 yellow ones.
Consequently these stand in the ratio
of 2.82:1.
Expt. 6. Position of
flowers. -- Among 858 cases 651 had
inflorescences axial and 207 terminal.
Ratio, 3.14:1.
Expt. 7. Length of
stem. -- Out of 1064 plants, in 787 cases the
stem was long, and in 277 short. Hence
a mutual ratio of 2.84:1.
In this
experiment the dwarfed plants were carefully
lifted and
transferred to a special
bed. This precaution was necessary, as
otherwise
they
would
have
perished
through
being
overgrown
by
their
tall
relatives.
Even
in
their
quite
young state
they
can
be
easily
picked out by their compact growth and
thick dark-green foliage.
If now the
results of the whole of the experiments be brought
together,
there is found, as between
the number of forms with the dominant and
recessive characters, an average ratio
of 2.98:1, or 3:1.
The
dominant
character
can
have
here
a
double
signification
;
namely,
that
of a
parental character, or a hybrid-character. In
which of the two
significations it
appears in each separate case can only be
determined
by the following generation.
As a parental character it must pass over
unchanged to the whole of the
offspring; as a hybrid-character, on the
other
hand,
it
must
maintain
the
same
behavior
as
in
the
first
generation.
[6] The Second
Generation From the Hybrids
Those
forms
which
in
the
first
generation
exhibit
the
recessive
character
do not further
vary in the second generation as regards this
character;
they remain constant in
their offspring.
It is otherwise with
those which possess the dominant character in the
first generation. Of these
two
-thirds yield offspring
which display the
dominant and
recessive characters in the proportion of 3:1, and
thereby
show exactly the same ratio as
the hybrid forms, while only
one
-third
remains
with the dominant character constant.
The separate experiments yielded the
following results:
第
10
页
共
57
页
Expt.
1.
Among
565
plants
which
were
raised
from
round
seeds
of
the
first generation, 193 yielded round
seeds only, and remained
therefore
constant
in
this
character;
372,
however,
gave
both
round
and
wrinkled seeds, in the proportion of 3:1. The
number of the
hybrids, therefore, as
compared with the constants is 1.93:1.
?
Expt. 2. Of 519
plants which were raised from seeds whose albumen
was
of
yellow
color
in
the
first
generation,
166
yielded
exclusively
yellow,
while
353
yielded
yellow
and
green
seeds
in
the
proportion
of 3:1. There
resulted, therefore, a division into hybrid and
constant forms in the proportion of
2.13:1.
?
For
each separate trial in the following experiments
100 plants were
selected
which
displayed
the
dominant
character
in
the
first
generation,
and
in
order
to
ascertain
the
significance
of
this,
ten
seeds
of
each
were
cultivated.
?
?
?
?
?
Expt.
3.
The
offspring
of
36
plants
yielded
exclusively
gray-brown
seed-coats,
while
of
the
offspring
of
64
plants
some
had
gray-
brown
and some had white.
Expt.
4.
The
offspring
of
29
plants
had
only
simply
inflated
pods;
of the offspring of
71, on the other hand, some had inflated and
some constricted.
Expt. 5.
The offspring of 40 plants had only green pods; of
the
offspring of 60 plants some had
green, some yellow ones.
Expt. 6.
The
offspring of
33
plants had only axial
flowers; of the
offspring
of
67,
on
the
other
hand,
some
had
axial
and
some
terminal
flowers.
Expt.
7.
The
offspring
of
28
plants
inherited
the
long
axis,
of
those
of 72 plants some the
long and some the short axis.
In
each
of
these
experiments
a
certain
number
of
the
plants
came
constant
with the dominant
character. For the determination of the proportion
in
which
the
separation
of
the
forms
with
the
constantly
persistent
character
results, the two
first experiments are especially important, since
in
these a larger number of plants can
be compared. The ratios 1.93:1 and
2.13:1 gave together almost exactly the
average ratio of 2:1. The sixth
experiment
gave
a
quite
concordant
results;
in
the
others
the
ratio
varies
more or less, as was
only to be expected in view of the smaller number
of 100 trial plants. Experiment 5,
which shows the greatest departure,
was
repeated,
and
then
in
lieu
of
the
ratio
of
60:40,
that
of
65:35
resulted.
The
average
ratio
of
2:1
appears,
therefore,
as
fixed
with
certainty.
It
is
therefore
demonstrated
that,
of
those
forms
which
posses
the
dominant
character
in
the
first
generation,
two-thirds
have
the
hybrid-
character,
while one-third remains
constant with the dominant character.
第
11
页
共
57
页
The
ratio
of
3:1,
in
accordance
with
which
the
distribution
of
the
dominant
and
recessive
characters
results
in
the
first
generation,
resolves
itself
therefore
in all experiments
into the ratio of 2:1:1,
if the
dominant
character be differentiated
according to its significance as a
hybrid-character or as a parental one.
Since the members of the first
generation spring directly
from
the seed of the
hybrids,
it is now clear
that
the
hybrids
form
seeds
having
one
or
other
of
the
two
differentiating
characters,
and of these one-half develop again the hybrid
form, while
the
other
half
yield
plants
which
remain
constant
and
receive
the
dominant
or the recessive
characters in equal numbers.
[7] The Subsequent Generations From
the Hybrids
The
proportions
in
which
the
descendants
of
the
hybrids
develop
and
split
up in the first and second generations
presumably hold good for all
subsequent
progeny.
Experiments
1
and
2
have
already
been
carried
through
6
generations, 3 and 7 through 5, and 4, 5, and 6
through 4, these
experiments
being
continued
from
the
third
generation
with
a
small
number
of
plants, and no departure from the rule has been
perceptible. The
offspring
of
the
hybrids
separated
in
each
generation
in
the
ratio
of
2:1:1
into hybrids and constant forms.
If
A
be
taken
as
denoting
one
of
the
two
constant
characters,
for
instance
the dominant,
a
the recessive, and
Aa
the hybrid form in which
both are
conjoined, the expression
A +
2Aa + a
shows the terms in
the series for the progeny of the hybrids of two
differentiating characters.
The
observation
made
by
G?rtner,
K?lreuter,
and
others,
that
hybrids
are
inclined to revert to the parental
forms, is also confirmed by the
experiments described. It is seen that
the number of the hybrids which
arise
from
one
fertilization,
as
compared
with
the
number
of
forms
which
become constant, and
their progeny from generation to generation, is
continually diminishing, but that
nevertheless they could not entirely
disappear. If an average equality of
fertility in all plants in all
generations be assumed, and if,
furthermore, each hybrid forms seed of
which
one-half
yields
hybrids
again,
while
the
other
half
is
constant
to
both
characters in equal proportions, the ratio of
numbers for the
offspring in each
generation is seen by the following summary, in
which
A
and
a
denote
again
the
two
parental
characters,
and
Aa
the
hybrid
forms.
For brevity's sake it may be assumed
that each plant in each generation
furnishes only 4 seeds.
第
12
页
共
57
页
Ratios
Generation
A Aa a
A : Aa : a
---------------------
-------------------------------
1
1 2 1
1 : 2 : 1
2
6 4 6
3 : 2 : 3
3
28 8
28 7 : 2 : 7
4
120 16 120 15 : 2 : 15
5 496 32 496 31 : 2 : 31
.
..........
........
n n
n
2 - 1 : 2 : 2 - 1
In the tenth generation, for instance,
2^
n
- 1 = 1023. There
result,
therefore, in each 2048 plants
which arise in this generation 1023 with
the constant dominant character, 1023
with the recessive character, and
only
two hybrids.
[8] The Offspring of
Hybrids in Which Several Differentiating
Characters are Associated.
In the experiments above described
plants were used which differed only
on
one essential character. The next task consisted
in ascertaining
whether the law of
development discovered in these applied to each
pair
of
differentiating
characters
when
several
diverse
characters
are
united
in the hybrid by
crossing. As regards the form of the hybrids in
these
cases,
the
experiments
showed
throughout
that
this
invariably
more
nearly
approaches to that one of the two
parental plants which possesses the
greater number of dominant characters.
If, for instance, the seed plant
has a
short stem, terminal white flowers, and simply
inflated pods; the
pollen plant, on the
other hand, a long stem, violet-red flowers
distributed along the stem, and
constricted pods; the hybrid resembles
the seed parent only in the form of the
pod; in the other characters it
agrees
with
the
pollen
parent.
Should
one
of
the
two
parental
types
possess
only dominant characters, then the
hybrid is scarcely or not at all
distinguishable from it.
Two experiments were made with a
considerable number of plants. In the
first
experiment
the
parental
plants
differed
in
the
form
of
the
seed
and
in the color of the
albumen; in the second in the form of the seed, in
the
color
of
the
albumen,
and
in
the
color
of
the
seed-
coats.
Experiments
with
seed
characters
give
the
result
in
the
simplest
and
most
certain
way.
第
13
页
共
57
页
In order to
facilitate study of the data in these experiments,
the
different
characters
of
the
seed
plant
will
be
indicated
by
A
,
B
,
C
,
those
of the pollen plant by
a
,
b
,
c
, and the hybrid forms of
the characters
by
Aa
,
Bb
, and
Cc
.
First
Experiment:
AB
Seed parents,
abc
Pollen parents,
A
form round
a
form wrinkled
B
albumen yellow
b
albumen
green
The fertilized seeds appeared
round and yellow like those of the seed
parents. The plants raised therefrom
yielded seeds of four sorts, which
frequently
presented
themselves
in
one
pod.
In
all,
556
seeds
were
yielded
by 15 plants, and of
these there were:
315 round and
yellow,
?
101
wrinkled and yellow,
?
108 round and green,
?
32 wrinkled and
green.
?
All
were sown the following year. 11 of the round
yellow seeds did not
yield plants, and
3 plants did not form seeds. Among the rest:
38 had round yellow seeds ........
AB
?
65 round yellow and green
seeds..........
ABb
?
60 round yellow
and wrinkled yellow
seeds........
AaB
?
138 round
yellow and green, wrinkled yellow
and
green seeds...... .....
AaBb
?
From the
wrinkled yellow seeds 96 resulting plants bore
seed, of which:
28 had only wrinkled
yellow
seeds................
aB
?
68 wrinkled
yellow and green seeds
.............
aBb
?
From 108 round
green seeds 102 resulting plants fruited, of
which:
35 had only round green seeds
...............
Ab
?
67 round and
wrinkled green seeds
..........
Aab
?
The wrinkled
green seeds yielded 30 plants which bore seeds all
of like
character; they remained
constant
ab
.
The
offspring
of
the
hybrids
appeared
therefore
under
9
different
forms,
some of them in very unequal numbers.
When these are collected and
coordinated we find:
第
14
页
共
57
页
38 plants with the sign
AB
35
28
30
65
68
60
67
138
The
whole
of
the
forms
may
be
classed
into
3
essentially
different
groups.
The
first
includes
those
with
the
signs
AB
,
Ab
,
aB
,
and
ab
:
they
possess
only
constant characters and do not vary again in the
next generation.
Each of these forms is
represented on the average 33 times. The second
group includes the signs
ABb
,
aBb
,
AaB
,
Aab
: these are constant in
one
character and hybrid in another,
and vary in the next
generation only as
regards
the hybrid-character. Each of these appears on any
average 65
times. The form
AaBb
occurs 138
times : it is hybrid in both characters,
and behaves exactly as do the hybrids
from which it is derived.
If the
numbers in which the forms belonging to these
classes appear be
compared, the ratios
of 1:2:4 are unmistakably evident. The numbers 33,
65, 138 present very fair
approximations to the
ratio
numbers of 33,
66,
132.
The development series consists,
therefore, of 9 classes, of which 4
appear
therein
always
once
and
are
constant
in
both
characters;
the
forms
AB
,
ab
,
resemble
the
parental
forms,
the
two
others
present
combinations
between the
conjoined characters
A
,
a
,
B
,
b
, which combinations are
likewise possibly constant. Four
classes appear always twice, and are
constant
in
one
character
and
hybrid
in
the
other.
One
class
appears
four
times, and is hybrid in both
characters. Consequently, the offspring of
the hybrids, if two kinds of
differentiating characters are combined
therein, are represented by the
expression
AB + Ab + aB +
ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb
This expression is indisputably a
combination series in which the two
expressions for the characters
A
and
a
,
B
and
b
are combined. We
arrive
at
the
full
number
of
the
classes
of
the
series
by
the
combination
of
the
expressions:
A + 2Aa + a
B +
2Bb + b
Second Experiment:
ABC
Seed parents,
abc
Pollen parents,
A
form round
a
form wrinkled
B
albumen yellow
b
albumen green
C
seed-coat gray-brown
c
seed-coat white
第
15
页
共
57
页
This experiment was made in precisely
the same way as the previous one.
Among
all the experiments
it demanded the
most
time and trouble.
From
24
hybrids 687 seeds were obtained in
all: these were all either spotted,
gray-brown
or
gray-green,
round
or
wrinkled.
From
these
in
the
following
year
639
plants
fruited,
and
as
further
investigation
showed,
there
were
among them:
8 plants ABC
22 plants ABCc
45 plants
ABbCc
14
17
9
25
11
20
40
8
15
10
18
48
10
19
7
24
14
78
18
20
16
The whole expression contains 27 terms.
Of these 8 are constant in all
characters, and each appears on the
average 10 times; 12 are constant in
two characters, and hybrid in the
third; each appears on the average 19
times; 6 are constant in one character
and hybrid in the other two; each
appears on the average 43 times. One
form appears 78 times and is hybrid
in
all of the characters. The ratios 10:19:43:78
agree so closely with
the
ratios
10:20:40:80,
or
1:2:4:8
that
this
last
undoubtedly
represents
the true value.
The
development of the hybrids when the original
parents differ in 3
characters results
therefore according to the following expression:
ABC + ABc + AbC + Abc + aBC
+ aBc + abC + abc +
2ABCc +
2AbCc + 2aBCc + 2abCc + 2ABbC + 2ABbc +
2aBbC + 2aBbc + 2AaBC +
2AaBc + 2AabC + 2Aabc +
4ABbCc + 4aBbCc + 4AaBCc + 4AabCc +
4AaBbC +
4AaBbc + 8AaBbCc.
Here also is involved a combination
series in which the expressions for
the
characters
A
and
a
,
B
and
b
,
C
and
c
, are united. The
expressions
A + 2Aa + a
B + 2Bb +
b
C + 2Cc + c
give
all
the
classes
of
the
series.
The
constant
combinations
which
occur
therein agree with all
combinations which are possible between the
第
16
页
共
57
页
characters
A
,
B
,
C
,
a
,
b
,
c
; two thereof,
ABC
and
abc
, resemble the two
original parental stocks.
In addition, further experiments were
made with a smaller number of
experimental
plants
in
which
the
remaining
characters
by
twos
and
threes
were
united
as
hybrids:
all
yielded
approximately
the
same
results.
There
is therefore no doubt
that for the whole of the characters involved in
the experiments the principle applies
that the offspring of the hybrids
in which several essentially different
characters are combined exhibit
the
terms
of
a
series
of
combinations,
in which
the
developmental
series
for
each
pair
of
differentiating
characters
are
united.
It
is
demonstrated
at the same
time that
the relation of each pair of
different characters
in
hybrid
union
is
independent
of
the
other
differences
in
the
two
original
parental stocks
.
If
n
represent
the number of the differentiating characters in
the two
original
stocks,
3^
n
gives
the
number
of
terms
of
the
combination
series,
4^
n
the number of
individuals which belong to the series, and
2^
n
the
number of
unions which remain constant. The series therefore
contains,
if the original stocks differ
in four characters, 3^
4
= 81
classes, 4^
4
=
256 individuals, and 2^
4
=
16 constant forms: or, which is the same,
among each 256 offspring of the hybrids
are 81 different combinations,
16 of
which are constant.
All constant
combinations which in Peas are possible by the
combination
of the said 7
differentiating characters were actually obtained
by
repeated crossing. Their number is
given by 2^
7
= 128. Thereby
is
simultaneously given the practical
proof
that the constant characters
which
appear
in
the
several
varieties
of
a
group
of
plants
may
be
obtained
in all the associations which are
possible according to the laws of
combination, by means of repeated
artificial fertilization.
As
regards
the
flowering
time
of
the
hybrids,
the
experiments
are
not
yet
concluded.
It
can,
however,
already
be
stated
that
the
time
stands
almost
exactly between those of the seed and
pollen parents, and that the
constitution of the hybrids with
respect to this character probably
follows
the
rule
ascertained
in
the
case
of
the
other
characters.
The
forms
which are selected for
experiments of this class must have a difference
of at least 20 days from the middle
flowering period of one to that of
the
other;
furthermore,
the
seeds
when
sown
must
all
be
placed
at
the
same
depth in
the earth, so that they may germinate
simultaneously. Also,
during the whole
flowering period, the more important variations in
temperature must be taken into account,
and the partial hastening or
delaying
of the flowering which may result therefrom. It is
clear that
第
17
页
共
57
页
this experiment presents many
difficulties to be overcome and
necessitates great attention.
If
we
endeavor
to
collate
in
a
brief
form
the
results
arrived
at,
we
find
that those
differentiating characters, which admit of easy
and certain
recognition
in
the
experimental
plants,
all
behave
exactly
alike
in
their
hybrid associations.
The
offspring of the hybrids of each pair of
differentiating characters are, one-
half, hybrid again, while the other
half
are
constant
in equal
proportions
having
the
characters
of
the
seed
and pollen parents
respectively. If several differentiating
characters
are
combined
by
cross-fertilization
in
a
hybrid,
the
resulting
offspring
form the terms of
a combination series in which the combination
series
for each pair of differentiating
characters are united.
The
uniformity
of
behavior
shown
by
the
whole
of
the
characters
submitted
to experiment
permits, and fully justifies, the acceptance of
the
principle that a similar relation
exists in the other characters which
appear
less
sharply
defined
in
plants,
and
therefore
could
not
be
included
in the separate experiments. An
experiment with peduncles of different
lengths gave on the whole a fairly
satisfactory results, although the
differentiation
and
serial
arrangement
of
the
forms
could
not
be
effected
with that certainty
which is indispensable for correct experiment.
[9] The Reproductive Cells of the
Hybrids
The results of the previously
described experiments led to further
experiments,
the
results
of
which
appear
fitted
to
afford
some
conclusions
as regards the
composition of the egg and pollen cells of
hybrids. An
important clue is afforded
in
Pisum
by the circumstance
that among the
progeny
of
the
hybrids
constant
forms
appear,
and
that
this
occurs,
too,
in respect of all combinations of the
associated characters. So far as
experience
goes,
we
find
it
in
every
case
confirmed
that
constant
progeny
can only be formed when the egg cells
and the fertilizing pollen are of
like
character,
so
that
both
are
provided
with
the
material
for
creating
quite similar
individuals, as is the case with the normal
fertilization
of pure species. We must
therefore regard it as certain that exactly
similar factors must be at work also in
the production of the constant
forms
in
the
hybrid
plants.
Since
the
various
constant
forms
are
produced
in
one
plant, or even in
one
flower of a plant, the
conclusion appears
logical
that
in
the
ovaries
of
the
hybrids
there
are
formed
as
many
sorts
of egg cells, and in
the anthers as many sorts of pollen cells, as
there
are possible constant combination
forms, and that these egg and pollen
cells agree in their internal
compositions with those of the separate
forms.
第
18
页
共
57
页
In point of
fact it is possible to demonstrate theoretically
that this
hypothesis would fully
suffice to account for the development of the
hybrids in the separate
generations, if we
might at
the
same time assume
that
the
various
kinds
of
egg
and
pollen
cells
were
formed
in
the
hybrids
on the average in
equal numbers.
In order to bring these
assumptions to an experimental proof, the
following experiments were designed.
Two forms which were constantly
different
in
the
form
of
the
seed
and
the
color
of
the
albumen
were
united
by fertilization.
If the differentiating characters are
again indicated as
A
,
B
,
a
,
b
, we
have:
AB
Seed parents,
ab
Pollen parents,
A
form round
a
form wrinkled
B
albumen yellow
b
albumen green
The artificially fertilized seeds were
sown together with several seeds
of
both original stocks, and the most vigorous
examples were chosen for
the reciprocal
crossing. There were fertilized:
1.
The hybrids
with the pollen of
AB
2.
The hybrids
with the pollen of
ab
3.
AB
with the pollen of the hybrids.
4.
ab
with the pollen of the hybrids.
For
each
of
these
4
experiments
the
whole
of
the
flowers
on
3
plants
were
fertilized. If the above theory be
correct, there must be developed on
the
hybrids
egg
and
pollen
cells
of
the
forms
AB
,
Ab
,
aB
,
ab
,
and
there
would
be
combined:
1.
The
egg cells
AB
,
Ab
,
aB
,
ab
with the pollen cells
AB
.
2.
The egg cells
AB
,
Ab
,
aB
,
ab
with the pollen cells
ab
.
3.
The egg cells
AB
with the pollen cells
AB
,
Ab
,
aB
, and
ab
.
4.
The egg cells
ab
with the pollen cells
AB
,
Ab
,
aB
, and
ab
.
From
each
of
these
experiments
there
could
then
result
only
the
following
forms:
1.
AB
,
ABb
,
AaB
,
AaBb
2.
AaBb
,
Aab
,
aBb
,
ab
3.
AB
,
ABb
,
AaB
,
AaBb
4.
AaBb
,
Aab
,
aBb
,
ab
第
19
页
共
57
页
If,
furthermore, the several forms of the egg and
pollen cells of the
hybrids were
produced on an average in equal numbers, then in
each
experiment
the
said
4
combinations
should
stand
in
the
same
ratio
to
each
other. A perfect
agreement in the numerical relations was, however,
not
to be expected since in each
fertilization, even in normal cases, some
egg cells remain undeveloped or
subsequently die, and many even of the
well-formed seeds fail to germinate
when sown. The above assumption is
also
limited in so far that while it demands the
formation of an equal
number of the
various sorts of egg and pollen cells, it does not
require
that this should apply to each
separate hybrid with mathematical
exactness.
The
first
and
second
experiments
had
primarily
the
object
of
proving
the
composition of the
hybrid egg cells, while the
third and
fourth
experiments were to
decide that of the pollen cells. As is shown by
the
above demonstration the first and
third experiments and the second and
fourth experiments should produce
precisely the same combinations, and
even
in
the
second
year
the
result
should
be
partially
visible
in
the
form
and color of the artificially
fertilized seed. In the first and third
experiments the dominant characters of
form and color,
A
and
B
, appear
in each
union, and are also partly constant and partly in
hybrid union
with
the
recessive
characters
a
and
b
,
for
which
reason
they
must
impress
their
peculiarity
upon
the
whole
of
the
seeds.
all
seeds
should
therefore
appear round and
yellow, if the theory be justified. In the second
and
fourth experiments, on the other
hand, one union is hybrid in form and
in color, and consequently the seeds
are round and yellow; another is
hybrid
in
form,
but
constant
in
the
recessive
character
of
color,
whence
the seeds are round and green; the
third is constant in the recessive
character
of
form
but
hybrid
in
color,
consequently
the
seeds
are
wrinkled
and
yellow;
the
fourth
is
constant
in
both
recessive
characters,
so
that
the seeds
are wrinkled and green. In both these experiments
there were
consequently
four
sorts
of
seed
to
be
expected;
namely,
round
and
yellow,
round and green,
wrinkled and yellow, wrinkled and green.
The crop fulfilled these expectations
perfectly. There were obtained in
the
1st Experiment, 98 exclusively round
yellow seeds;
?
3rd Experiment, 94 exclusively round
yellow seeds.
?
In
the
2nd
Experiment,
31
round
and
yellow,
26
round
and
green,
27
wrinkled
and yellow, 26
wrinkled and green seeds.
In
the
4th
Experiment,
24
round
and
yellow,
25
round
and
green,
22
wrinkled
and yellow, 27
wrinkled and green seeds.
第
20
页
共
57
页
There could
scarcely be now any doubt of the success of the
experiment;
the
next
generation
must
afford
the
final
proof.
From
the
seed
sown
there
resulted for the
first
experiment
90 plants,
and for the
third 87 plants
which fruited: these yielded for the
1st Exp. 3rd Exp.
20
25
round yellow seeds
..................... AB
23
19
round yellow and green
seeds ............. ABb
25
22
round and wrinkled
yellow seeds .......... AaB
22
21
round and wrinkled
green and yellow seeds.. AaBb
In the
second and fourth experiments the round and yellow
seeds yielded
plants with round and
wrinkled yellow and green seeds,
AaBb
.
From
the
round
green
seeds
plants
resulted
with
round
and
wrinkled
green
seeds,
Aab
.
The
wrinkled
yellow
seeds
gave
plants
with
wrinkled
yellow
and
green
seeds,
aBb
.
From
the
wrinkled
green
seeds
plants
were
raised
which
yielded
again
only
wrinkled and green
seeds,
ab
.
Although
in
these
two
experiments
likewise
some
seeds
did
not
germinate,
the figures
arrived at already in the previous year were not
affected
thereby,
since
each
kind
of
seed
gave
plants
which,
as
regards
their
seed,
were
like each other and different from the others.
There resulted
therefore from the
2nd. Exp. 4th Exp.
31
24
plants of the form AaBb
26
25
plants of the form Aab
27
22
plants of the form aBb
26
27
plants of the form ab
In
all
the
experiments,
therefore,
there
appeared
all
the
forms
which
the
proposed theory demands, and they came
in nearly equal numbers.
In
a
further
experiment
the
characters
of
flower-color
and
length
of
stem
were
experimented upon,
and
selection was
so
made that in
the third year
of
the
experiment
each
character
ought
to
appear
in
half
of
all
the
plants
if the above theory
were correct.
A
,
B
,
a
,
b
serve again as indicating the
various characters.
A,
violet-red flowers;
?
a, white flowers;
?
B, axis long;
?
第
21
页
共
57
页
?
A, axis short.
The form
Ab
was fertilized with
ab
, which produced the
hybrid
Aab
.
Furthermore,
aB
was also fertilized with
ab
,
whence the hybrid
aBb
. In
the second year, for further
fertilization, the hybrid
Aab
was used as
seed parent, and hybrid
aBb
as pollen parent.
Seed parent,Aab;
?
Pollen
parentaBb;
?
Possible egg cells, Ab,ab;
?
Pollen cells,
aB, ab.
?
From
the fertilization between the possible egg and
pollen cells four
combinations should
result, namely:
?
AaBb
+
aBb
+
Aab
+
ab
From
this
it
is
perceived
that,
according
to
the
above
theory,
in
the
third
year of
the experiment out of all the plants
half should have violet-red flowers (Aa)
Classes 1, 3
1, 2
From 45 fertilizations of the second
year 187 seeds resulted, of which
only
166 reached the flowering stage in the third year.
Among these the
separate classes
appeared in the numbers following:
Class Flower color Stem
---------------------------------------------
1
violet-red
long
47 times
2
white
long
40
3
violet-red
short 38
4
white
short
41
There subsequently appeared
the violet-red flower color
(
Aa
) in 85 plants,
?
the white
flower-color (
a
) in 81
plants,
?
the
long stem (
Bb
) in 87 plants,
?
the short stem
(
b
) in 79 plants.
?
The theory
adduced is therefore satisfactorily confirmed in
this
experiment also.
For
the
characters
of
form
of
pod
,
color
of
pod
,
and
position
of
flowers
,
experiments
were
also
made
on
a
small
scale
and
results
obtained
in
perfect
第
22
页
共
57
页
agreement.
All
combinations,
which
were
possible
through
the
union
of
the
differentiating
characters duly appeared, and in nearly equal
numbers.
Experimentally, therefore,
the theory is confirmed that
the pea
hybrids
form
egg
and
pollen
cells
which,
in
their
constitution,
represent
in
equal
numbers
all constant forms which result from the
combination of the
characters united in
fertilization
.
The
difference of the forms among the progeny of the
hybrids, as well as
the respective
ratios of the numbers in which they are observed,
find a
sufficient
explanation
in
the
principle
above
deduced.
The
simplest
case
is afforded by the developmental series
of
each pair of differentiating
characters
.
This
series
is
represented
by
the
expression
A+2Aa+a
,
in
which
A
and
a
signify the forms with
constant differentiating characters, and
Aa
the hybrid form of both.
It includes in 3 different classes 4
individuals. In the formation
of these, pollen
and egg
cells
of the form
A
and
a
take
part
on
the
average
equally
in
the
fertilization;
hence
each
form
[occurs]
twice,
since
four
individuals
are
formed.
There
participate
consequently in
the fertilization
the pollen cells
A
+
A
+
a
+
a
,
o
the egg cells <
/p>
A
+
A
+
a
+
a
.
o
It remains,
therefore, purely a matter of chance which of the
two sorts
of pollen will become united
with each separate egg cell. According,
however,
to
the
law
of
probability,
it
will
always
happen,
on
the
average
of
many
cases,
that
each
pollen
form
A
and
a
will
unite
equally
often
with
each
egg cell form
A
and
a
, consequently one of the
two pollen cells
A
in
the
fertilization
will
meet
with
the
egg
cell
A
and
the
other
with
the
egg
cell
a
,
and
so
likewise
one
pollen
cell
a
will
unite
with
an
egg
cell
A
, and the other with the
egg cell
a
.
Pollen cells
A
A
a
a
|
/ |
|
X
|
|
/
|
Egg
cells
A A a a
The result of the fertilization may be
made clear by putting the signs
for the
conjoined egg and pollen cells in the form of
fractions, those
for the pollen cells
above and those for the egg cells below the line.
We then have
A
A
a a
----- + ----- +
----- + -----
A
a
A a
In
the first and fourth term the egg and pollen cells
are of like kind,
consequently the
product of their union must be constant, namely
A
and
第
23
页
共
57
页
a
;
in
the
second
and
third,
on
the
other
hand,
there
again
results
a
union
of the two
differentiating characters of the stocks,
consequently the
forms
resulting
from
these
fertilizations
are
identical
with
those
of
the
hybrid from which they
sprang.
There occurs accordingly a
repeated
hybridization.
This
explains
the
striking
fact
that
the
hybrids
are
able
to
produce, besides the two parental forms, offspring
which are like
themselves;
A
a
----- and -----
a
A
both give the same union
Aa
, since, as already
remarked above, it makes
no
difference
in
the
result
of
fertilization
to
which
of
the
two
characters
the pollen or egg cells belong. We may
write then
A
A
a
a
--- +
--- + --- + --- = A + 2Aa + a
A
a
A
a
This represents the
average result of the self-fertilization of the
hybrids when two differentiating
characters are united in them. In
individual
flowers
and
in
individual
plants,
however,
the
ratios
in
which
the forms of the
series are produced may suffer not inconsiderable
fluctuations. Apart from the fact that
the numbers in which both sorts
of egg
cells occur in the seed vessels can only be
regarded as equal on
the average, it
remains purely a matter of chance which of the two
sorts
of pollen may fertilize each
separate egg cell. For this reason the
separate values must necessarily be
subject to fluctuations, and there
are
even
extreme
cases
possible,
as
were
described
earlier
in
connection
with
the
experiments
on
the
forms
of
the
seed
and
the
color
of
the
albumen.
The true ratios of the numbers can only
be ascertained by an average
deduced
from the sum of as many single values as possible;
the greater
the number the more are
merely chance effects eliminated.
The
developmental
series
for
hybrids
in
which
two
kinds
of
differentiating
characters
are
united
contains
among
16
individuals
9
different
forms,
AB
+ Ab +
aB + ab + 2ABb + 2aBb + 2AaB + 2Aab +
4AaBb
. Between the
differentiating characters of the
original stocks
Aa
and
Bb
4 constant
combinations are possible, and
consequently the hybrids produce the
corresponding 4 forms of egg and pollen
cells
AB
,
Ab
,
aB
,
ab
, and each of
these will on the
average figure 4 times in the fertilization, since
16
individuals are included in the
series. Therefore, the participators in
the fertilization are
Pollen cells:
AB+AB+AB+AB+Ab+Ab+Ab+Ab+aB+aB+aB+aB+ab+ab+ab+ab.
?
Egg cells:
AB+AB+AB+AB+Ab+Ab+Ab+Ab+aB+aB+aB+aB+ab+ab+ab+ab.
?
第
24
页
共
57
页
In the process
of fertilization each pollen form unites on an
average
equally
often
with
each
egg
cell
form,
so
that
each
of
the
4
pollen
cells
AB
unites
once
with
one
of
the
forms
of
egg
cell
AB
,
Ab
aBab
.
In
precisely
the same way the
rest of the pollen cells of the forms
AbaBab
unite with
all the other egg cells. We obtain
therefore
AB
AB AB AB Ab
Ab
Ab
Ab
---- + ---- + ---- + ---- + ---- + ----
+ ---- + ----
AB
Ab aB ab AB Ab aB
ab
aB aB
aB
aB ab
ab ab
ab
+ ---- + ---- + ---- + ---- + ---- +
---- + ---- + ----
AB
Ab aB ab AB Ab aB ab
or
?
AB + ABb + AaB + AaBb + ABb + Ab + AaBb
+ Aab + AaB + AaBb
+ aB + aBb + AaBb +
Aab + aBb + ab
=
AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB
+ 2Aab + 4AaBb
In precisely
similar fashion is the developmental series of
hybrids
exhibited when
three
kinds of differentiating characters
are
conjoined
in them. The hybrids form 8
various kinds of egg and pollen cells:
ABC
,
ABc
,
AbC
,
Abc
,
aBC
,
aBc
,
abC
,
abc
;
and
each
pollen
form
unites
itself
again
on the average once
with each form of egg cell.
The law of
combination of different characters which governs
the
development
of
the
hybrids
finds
therefore
its
foundation
and
explanation
in
the
principle
enunciated,
that
the
hybrids
produce
egg
cells
and
pollen
cells
which in equal numbers represent all constant
forms which result
from the
combinations of the characters brought together in
fertilization.
[10]
Experiments with Hybrids of Other Species of
Plants
It
must
be
the
object
of
further
experiments
to
ascertain
whether
the
law
of
development
discovered
for
Pisum
applies
also
to
the
hybrids
of
other
plants.
To
this
end
several
experiments
were
recently
commenced.
Two
minor
experiments
with
species
of
Phaseolus
have
been
completed,
and
may
be
here
mentioned.
An experiment with Phaseolus vulgaris
and Phaseolus nanus gave results
in
perfect
agreement.
Ph.
nanus
had
together
with
the
dwarf
axis,
simply
inflated,
green
pods.
Ph.
vulgaris
had,
on
the
other
hand,
an
axis
10
ft.
第
25
页
共
57
页
to
12
ft.
high,
and
yellow
colored
pods,
constricted
when
ripe.
The
ratios
of the numbers in which the different
forms appeared in the separate
generations were the same as with
Pisum. Also the development of the
constant
combinations
resulted
according
to
the
law
of
simple
combination
of characters,
exactly as in the case of Pisum. There were
obtained
Constant
Axis
Color of the
Form of
the
combinations
unripe pods ripe pods
--------
-------------------------------------------------
1
long
green
inflated
2
constricted
3
yellow
inflated
4
5
short
green
inflated
6
constricted
7
yellow
inflated
8
constricted
The green color of the pod, the
inflated forms, and the long axis were,
as in Pisum, dominant characters.
Another
experiment
with
two
very
different
species
of
Phaseolus
had
only
a
partial
result.
Phaseolus
nanus,
L,
served
as
seed
parent
,
a
perfectly
constant species, with white
flowers in short recemes and small white
seeds in straight, inflated, smooth
pods; as
pollen parent
was
used Ph.
multiflorus, W, with tall
winding stem, purple-red flowers in very long
recemes, rough, sickle-shaped crooked
pods, and large seeds which bore
black
flecks and splashes on a peach-blood-red ground.
The hybrids had the greatest similarity
to the pollen parent, but the
flowers
appeared
less
intensely
colored.
Their
fertility
was
very
limited;
from 17 plants, which together
developed many hundreds of flowers, only
49
seeds
in
all
were
obtained.
These
were
of
medium
size,
and
were
flecked
and
splashed
similarly
to
those
of
Ph.
multiflorus,
while
the
ground
color
was not materially
different. The next year 44 plants were raised
from
these
seeds,
of
which
only
31
reached
the
flowering
stage.
The
characters
of
Ph.
nanus,
which
had
been
altogether
latent
in
the
hybrids,
reappeared
in various
combinations; their ratio, however, with relation
to the
dominant
plants
was
necessarily
very
fluctuating
owing
to
the
small
number
of
trial
plants.
With
certain
characters,
as
in
those
of
the
axis
and
the
form
of
pod,
it
was,
however,
as
in
the
case
of
Pisum,
almost
exactly
1:3.
Insignificant as the
results of this experiment may be as regards the
determination
of
the
relative
numbers
in
which
the
various
forms
appeared,
it presents, on
the
other hand,
the
phenomenon of a
remarkable
change of
color
in the
flowers and seed of the hybrids. In Pisum it is
known that
第
26
页
共
57
页
the
characters
of
the
flower-
and
seed-color
present
themselves
unchanged
in
the
first
and
further
generations,
and
that
the
offspring
of
the
hybrids
display
exclusively
the
one
or
the
other
of
the
characters
of
the
original
stocks. It is
otherwise in the experiment we are considering.
The white
flowers and the seed-color of
Ph. nanus appeared, it is true, at once in
the
first
generation in
one
fairly
fertile
example,
but
the
remaining
30
plants
developed
flower-colors
which
were
of
various
grades
of
purple-
red
to
pale
violet.
The
coloring
of
the
seed-coat
was
no
less
varied
than
that
of the flowers. No
plant could rank as fully fertile; many produced
no
fruit
at
all;
others only
yielded
fruits
from
the
flowers
last produced,
which did not ripen. From 15 plants
only were well-developed seeds
obtained. The greatest disposition to
infertility was seen in the forms
with
preponderantly red
flowers, since out
of 16 of these only
4
yielded
ripe seed. Three of
these had
a
similar seed pattern to Ph.
multiflorus,
but with a more or less
pale ground color; the fourth plant yielded only
one
seed
of
plain
brown
tint.
The
forms
with
preponderantly
violet-
colored
flowers had dark brown, black-
brown, and quite black seeds.
The
experiment was continued through two more
generations under similar
unfavorable
circumstances, since even among the offspring of
fairly
fertile
plants
there
came
again
some
which
were
less
fertile
and
even
quite
sterile. Other flower- and seed-colors
than those cited did not
subsequently
present
themselves.
The
forms
which
in
the
first
generation
contained one or
more of the recessive characters remained, as
regards
these, constant without
exception. Also of those plants which possessed
violet flowers and brown or black seed,
some did not vary again in these
respects
in
the
next
generation;
the
majority,
however,
yielded
together
with offspring
exactly like themselves, some which displayed
white
flowers and white seed-coats. The
red flowering plants remained so
slightly
fertile
that
nothing
can
be
said
with
certainty
as
regards
their
further development.
Despite the many disturbing factors
with which the observations had to
contend,
it
is
nevertheless
seen
by
this
experiment
that
the
development
of the hybrids, with regard to those
characters which concern the form
of
the
plants,
follows
the
same
laws
as
in
Pisum.
With
regard
to
the
color
characters, it
certainly appears difficult to perceive a
substantial
agreement. Apart from the
fact that from the union of a white and a
purple-red
coloring
a
whole
series
of
colors
results,
from
purple
to
pale
violet and white, the circumstance is a
striking one that among 31
flowering
plants only one received the recessive character
of the white
color, while in Pisum this
occurs on the average in every fourth plant.
Even these enigmatical results,
however, might probably be explained by
the law governing Pisum if we might
assume that the color of the flowers
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and seeds of Ph. multiflorus is a
combination of two or more entirely
independent colors, which individually
act like any other constant
character
in the plant. If the flower-color
A
were a combination of the
individual characters
A(1) +
A(2) + .....
which produce the total
impression of a purple coloration, then
by fertilization with the
differentiating character, white color,
a
, there would be produced
the
hybrid unions
A(1)a +
A(2)a + .....
and so would it be with
the
corresponding coloring of the seed-
coats. According to the above
assumptions,
each
of
these
hybrid
color
unions
would
be
independent,
and
would
consequently
develop
quite
independently
from
the
others.
It
is
then
easily
seen
that
from
the
combination
of
the
separate
developmental
series
a complete color-series must result.
If, for instance,
A = A(1) +
A(2)
,
then the hybrids
A(1)a
and
A(2)a
form the developmental
series:
A(1) +
2A(1)a + a
A(2) + 2A(2)a + a
The members of this series can enter
into nine different combinations,
and
each of these denotes another color:
1 A(1)A(2)
2
A(1)aA(2)
1 A(2)a
2 A(1)A(2)a
4
A(1)aA(2)a
2 A(2)aa
1 A(1)a
2 A(1)aa
1 aa
The
figures prescribed for the separate combinations
also indicate how
many plants with the
corresponding coloring belong to the series. Since
the total is 16, the whole of the
colors are on the average distributed
over each 16 plants, but, as the series
itself indicated, in unequal
proportions.
Should the
color development really happen in this way, we
could offer
an explanation of the case
above described, namely that of the white
flowers
and
seed-
coat
color
only
appeared
once
among
31
plants
of
the
first
generation. This coloring appears only
once in the series, and could
therefore
also
only
be
developed
once
in
the
average
in
each
16,
and
with
three color characters
only once even in 64 plants.
It must,
nevertheless, not be forgotten that the
explanation here
attempted is based on
a mere hypothesis, only supported by the very
imperfect
result
of
the
experiment
just
described.
It
would,
however,
be
well
worth while to follow up the development of color
in hybrids by
similar
experiments,
since
it
is
probable
that
in
this
way
we
might
learn
the significance of
the extraordinary variety in the
coloring of our
ornamental
flowers
.
So far, little at
present is known with certainty beyond the fact
that
the
color
of
the
flowers
in
most
ornamental
plants
is
an
extremely
variable
character.
The
opinion
has
often
been
expressed
that
the
stability
of
the
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